This invention relates to encoding of signals, and more specifically, to encoding of signals with a controlled quantizer.
When image signals are digitized and linearly quantized, a transmission rate of about 100 Mbits per second is necessary for images derived from standard TV signals. For HDTV, with its greater image size and greater resolution, a much higher transmission rate would be is required. When terrestrial transmission is desired, and when the option of allocating greater bandwidth per channel is unavailable, it is necessary to compress the HDTV signal within the allocated bandwidth. Having an allocated bandwidth is tantamount to having a certain number of information bits that can be transmitted in each selected time interval, such as an image frame. All of the compression techniques of HDTV signals must of necessity consider this bit budget.
In order to reduce the bit rate, or to fit the encoded signal of an image frame within the allocated bit budget, various coding schemes have been studied. One is the so-called "differential pulse code modulation" (DPCM) coding approach. By this method, the value of a particular pixel at any moment is predicted on the basis of values of pixels which have been already coded. The necessary number of bits is reduced by coding the difference (error) between the predicted value and the value of the particular pixel at that moment. According to this differential pulse code modulation, it is possible to reduce the number of bits per pixel by approximately half.
Still, that number is much larger than can be accepted for terrestrial transmission, so a second look is typically taken at the quantizer itself. Clearly, if the step size of the quantizer is made large enough, the number of bits generated can be reduced to an acceptable level. Alas, the quantization error resulting therefrom would yield a picture that is far from acceptable.
Experiments show, and it is quite logical, that the effect of quantization error is different for different types of picture and, expectedly, artisans have tried to tailor quantizers to the pictures being encoded.
U.S. Pat. No. 4,802,232 issued Jan. 31, 1989, for example, describes one such attempt. In accordance with the described approach the picture to be encoded is divided into regions and each region of the input image is classified into one of a preset number of classes. The signal of each class is quantized in a specified manner, and the manner of quantization of the different classes, of course, differ. The limitation of this approach is the hard decisions boundaries between the classes.
Nill in "A Visual Model Weighted Cosine Transform for Image Compression and Quality Assessment", IEEE Transactions on Communications, Vol. COM-33, No. 6, June, 1985, pg. 551-557 and Saghri et al. "Image quality measure based on a human visual system model", Optical Engineering, Vol. 28, No. 7, July 1989, pg. 813-818, describe very similar approaches. These approaches utilize fixed global thresholds for each frequency band, where the thresholds are set based on a model of the human visual systems (HVS) modulation transfer function (MTF), and the transform utilized. The limitation of this approach is that the quantization cannot adapt to local characteristics of the image.
In "Adaptive Quantization of Picture Signals using Spatial Masking", Proceedings of IEEE, Vol. 65, April 1977, pg. 536-548, Netravali et al. describe a means of designing non-uniform quantizers to incorporate spatial masking and brightness correction for predictive coding systems. The limitation of their approach, again, is that the quantization cannot adapt to local characteristics of the image.
In "Design of Statistically Based Buffer Control Policies for Compressed Digital Video", Zdepski et al., an IEEE conference, 1989, pg. 1343-1349, describe an approach where the quantizer in the DPCM loop interacts with an adaptive mode control circuit. The circuit measures the number of bits generated by the quantizer and, based on preselected thresholds, decides for the next frame on one of eight possible quantizer step sizes. The selected step size is employed for the next frame. A similar approach is described in "Digital Pictures" by A. N. Netravali and B. G. Haskell, Plenum Press, 1988, pg. 537 et seq.
The deficiency of these methods is that the quantization steps are altered in discrete jumps and in that no accounting is made of a global distortion target.
It is an object of this invention, therefore, to develop means for controlling the quantizer so that the number of bits generated per frame is, on the average, within the allocated bit budget.
It is another object of this invention to control the quantizer in a manner that is sensitive to the characteristics of the input signal to which human viewers are sensitive.
It is still another object of this invention to control the quantizer in a manner that spreads as evenly as possible the unavoidable quantization noise.